<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>async_compose</title>
<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../../boost_asio.html" title="Boost.Asio">
<link rel="up" href="../reference.html" title="Reference">
<link rel="prev" href="async_completion/result.html" title="async_completion::result">
<link rel="next" href="async_connect.html" title="async_connect">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td>
<td align="center"><a href="../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="async_completion/result.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="async_connect.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.async_compose"></a><a class="link" href="async_compose.html" title="async_compose">async_compose</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="boost_asio.indexterm.async_compose"></a> 
Launch an asynchronous operation
        with a stateful implementation.
      </p>
<pre class="programlisting">template&lt;
    typename CompletionToken,
    typename Signature,
    typename Implementation,
    typename... IoObjectsOrExecutors&gt;
<a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_compose(
    Implementation &amp;&amp; implementation,
    CompletionToken &amp; token,
    IoObjectsOrExecutors &amp;&amp;... io_objects_or_executors);
</pre>
<p>
        The async_compose function simplifies the implementation of composed asynchronous
        operations automatically wrapping a stateful function object with a conforming
        intermediate completion handler.
      </p>
<h5>
<a name="boost_asio.reference.async_compose.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.async_compose.parameters"></a></span><a class="link" href="async_compose.html#boost_asio.reference.async_compose.parameters">Parameters</a>
      </h5>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">implementation</span></dt>
<dd><p>
              A function object that contains the implementation of the composed
              asynchronous operation. The first argument to the function object is
              a non-const reference to the enclosing intermediate completion handler.
              The remaining arguments are any arguments that originate from the completion
              handlers of any asynchronous operations performed by the implementation.
            </p></dd>
<dt><span class="term">token</span></dt>
<dd><p>
              The completion token.
            </p></dd>
<dt><span class="term">io_objects_or_executors</span></dt>
<dd><p>
              Zero or more I/O objects or I/O executors for which outstanding work
              must be maintained.
            </p></dd>
</dl>
</div>
<h5>
<a name="boost_asio.reference.async_compose.h1"></a>
        <span class="phrase"><a name="boost_asio.reference.async_compose.example_"></a></span><a class="link" href="async_compose.html#boost_asio.reference.async_compose.example_">Example:</a>
      </h5>
<pre class="programlisting">struct async_echo_implementation
{
  tcp::socket&amp; socket_;
  boost::asio::mutable_buffer buffer_;
  enum { starting, reading, writing } state_;

  template &lt;typename Self&gt;
  void operator()(Self&amp; self,
      boost::system::error_code error = {},
      std::size_t n = 0)
  {
    switch (state_)
    {
    case starting:
      state_ = reading;
      socket_.async_read_some(
          buffer_, std::move(self));
      break;
    case reading:
      if (error)
      {
        self.complete(error, 0);
      }
      else
      {
        state_ = writing;
        boost::asio::async_write(socket_, buffer_,
            boost::asio::transfer_exactly(n),
            std::move(self));
      }
      break;
    case writing:
      self.complete(error, n);
      break;
    }
  }
};

template &lt;typename CompletionToken&gt;
auto async_echo(tcp::socket&amp; socket,
    boost::asio::mutable_buffer buffer,
    CompletionToken&amp;&amp; token) -&gt;
  typename boost::asio::async_result&lt;
    typename std::decay&lt;CompletionToken&gt;::type,
      void(boost::system::error_code, std::size_t)&gt;::return_type
{
  return boost::asio::async_compose&lt;CompletionToken,
    void(boost::system::error_code, std::size_t)&gt;(
      async_echo_implementation{socket, buffer,
        async_echo_implementation::starting},
      token, socket);
}
</pre>
<h5>
<a name="boost_asio.reference.async_compose.h2"></a>
        <span class="phrase"><a name="boost_asio.reference.async_compose.requirements"></a></span><a class="link" href="async_compose.html#boost_asio.reference.async_compose.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/compose.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
      M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="async_completion/result.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="async_connect.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
